The Information Paradox
August 26, 2015
One of the trending topics on various social media websites today has been the claim by Stephen Hawking that he has solved the information paradox. (To be fair to him, I believe he has just proposed a possible solution, and the media has exaggerated his claims for dramatic effect). And while what he has proposed has been suggested in other forms in the past, and other experts in the field disagree with the validity of his idea, it is still and interesting argument and one well worth exploring.
First we have to explain the information paradox.
Hopefully everyone who has sufficient interest to read this blog already knows what a black hole is. It is an object which is so dense that nothing can escape it. However as so often happens, quantum mechanics laughs at the impossible, and back in 1977 when Hawking and others applied quantum mechanical methods to black holes, then discovered that black holes can very slowly evaporate. (I won't give the details here, as I have written about this phenomena in depth in past articles)
However it has long been known that there is a critical problem with Hawking radiation. The matter that collapses to form the black hole contains a lot of information. Maybe you drop an encyclopedia full of information, or maybe the information is simply the chemical composition or the crystalline structure of the matter. The radiation that is emitted from the black hole cannot carry any information - it is purely random in nature. And according to the most fundamental laws of quantum mechanics, information cannot be destroyed in any process. It can be changed into other forms of information, but it can never be destroyed.
Which means that there are only three possible fates for the information in the black hole:
Option 1 is that black holes can leak information in some form. Maybe Hawking radiation is not quite random and a tiny amount of information is released back into the Universe. But that effectively means that black holes really are not black at all, and causes trouble for the theory of relativity as information is travelling faster than light to escape the black hole.
Option 2 is that the black hole evaporation stops at some point, and what is left is a lump of information forevermore hidden from the rest of the Universe. However there is nothing in the theory of Hawking radiation that allows for this end, and worse still the evaporation actually speeds up as the black hole gets smaller!
Option 3 is the ever present possibility that we know nothing about gravity at high energies. We know for a fact that the theory of general relativity breaks down at high energies and at very small scales, and while there are numerous candidates for a model of quantum gravity none has yet been proven to be true, or even complete. So maybe the answer is that the laws of physics we know are simply not applicable to black holes.
And for close to forty years no one has been able to solve this dilemma, or to provide a solid, consistent resolution of the paradox.
However Hawking today has advocated for a fourth option. Without going into the gory details of his newest model, what he is claiming is that as the matter enters the black hole it leaves its information content in a thin shell just outside of the event horizon. Since the information never enters the black hole, there is no problem with getting it out again.
Others are skeptical of this claim. To be blunt, if this model had been proposed by someone less famous in the popular culture (and indeed very similar models have been in the past), then no one outside of the specialists in this field would have even noticed the announcement. Trapping information before it falls in seems a bit of a workaround rather than a genuine solution.
However as with all new theories, it will take time to see if this one has any substance. Other experts are already studying it in detail, and without a doubt new papers expanding and dissecting Hawking's proposal will be published in the next few weeks. And only time will tell if it is a genuine solution to this fascinating puzzle, or just another failed idea in the history of theoretical physics.
First we have to explain the information paradox.
Hopefully everyone who has sufficient interest to read this blog already knows what a black hole is. It is an object which is so dense that nothing can escape it. However as so often happens, quantum mechanics laughs at the impossible, and back in 1977 when Hawking and others applied quantum mechanical methods to black holes, then discovered that black holes can very slowly evaporate. (I won't give the details here, as I have written about this phenomena in depth in past articles)
However it has long been known that there is a critical problem with Hawking radiation. The matter that collapses to form the black hole contains a lot of information. Maybe you drop an encyclopedia full of information, or maybe the information is simply the chemical composition or the crystalline structure of the matter. The radiation that is emitted from the black hole cannot carry any information - it is purely random in nature. And according to the most fundamental laws of quantum mechanics, information cannot be destroyed in any process. It can be changed into other forms of information, but it can never be destroyed.
Which means that there are only three possible fates for the information in the black hole:
Option 1 is that black holes can leak information in some form. Maybe Hawking radiation is not quite random and a tiny amount of information is released back into the Universe. But that effectively means that black holes really are not black at all, and causes trouble for the theory of relativity as information is travelling faster than light to escape the black hole.
Option 2 is that the black hole evaporation stops at some point, and what is left is a lump of information forevermore hidden from the rest of the Universe. However there is nothing in the theory of Hawking radiation that allows for this end, and worse still the evaporation actually speeds up as the black hole gets smaller!
Option 3 is the ever present possibility that we know nothing about gravity at high energies. We know for a fact that the theory of general relativity breaks down at high energies and at very small scales, and while there are numerous candidates for a model of quantum gravity none has yet been proven to be true, or even complete. So maybe the answer is that the laws of physics we know are simply not applicable to black holes.
And for close to forty years no one has been able to solve this dilemma, or to provide a solid, consistent resolution of the paradox.
However Hawking today has advocated for a fourth option. Without going into the gory details of his newest model, what he is claiming is that as the matter enters the black hole it leaves its information content in a thin shell just outside of the event horizon. Since the information never enters the black hole, there is no problem with getting it out again.
Others are skeptical of this claim. To be blunt, if this model had been proposed by someone less famous in the popular culture (and indeed very similar models have been in the past), then no one outside of the specialists in this field would have even noticed the announcement. Trapping information before it falls in seems a bit of a workaround rather than a genuine solution.
However as with all new theories, it will take time to see if this one has any substance. Other experts are already studying it in detail, and without a doubt new papers expanding and dissecting Hawking's proposal will be published in the next few weeks. And only time will tell if it is a genuine solution to this fascinating puzzle, or just another failed idea in the history of theoretical physics.
Posted In : Astronomy
I am a theoretical physicist & mathematician, with training in electronics, programming, robotics, and a number of other related fields.
